What should I use as a diorama base.

There are a few ways that you could do it:
1.) Wargamers method: layers of shaped foam boards covered with putty.
2.) Model RR method: chickenwire/wire mesh shaped and stapled to the base covered with plaster bandages/putty.

Those will give you your basic terrain shapes what you cover them with depends on what you want to represent.

As for basecoat/color I would go with earth/soil/dirt color.

The wire mesh makes for hollow terrain which makes it light-weight and allows for electronics, etc to be hidden, might also be good for other uses.

you can also use picture frames I think, or you could buy plaques or bases at some hobby store or a place where they make trophies or awards or something
--------------------------------------------------------------------------------------Kevin-------------------------

The styrfoam works the best for me, the model RR method is messy, and it can be hard to achieve the desired affect.